Many different kinds of multimode scanning and detection systems are currently known. Such systems may be active or passive in operation, being operationally effective in scanning or detecting multiple beams of radiation at multiple frequencies and wavelengths. The frequencies of operation include infrared radiation, in which heat is detected to identify a particular target or target region. Detection may be accomplished in the radar or radio frequency bands, accomplished either actively or passively, or with a combination of active and passive modes.
The term multimode can further be taken to refer to detection at one mode of energy operating at a given frequency, and also at another mode operating at the same or at a different frequency. Since several modes or frequencies of the electromagnetic spectrum are used, this approach is frequently referred to as multi-spectral. Multimode can further be taken to mean the use of both active and passive bands of radiation. It can further mean the use of one or more radar bands of radiation and one or more infrared bands. Multimode detection systems can moreover be ground based, ship based, airborne or set aloft in space.
In general, multimode detection systems enhance the detection flexibility and effectiveness of the system using the technique. For example, one beam can be designed to be wide in shape in order to conduct search operations, and the other beam can be narrow in order to accomplish tracking once the target has been identified. The different modes can relate to the distance or range of detection as well. For example, one mode can be used at short range and the other mode at extended ranges. In other words, the radar mode can be used at long ranges and the infrared detection can be employed closer in. The different modes also represent different operational capabilities. For example, radar can operate in poor weather while infrared techniques are limited under such conditions. However, infrared can provide significantly better resolution than radar, and can operate passively and therefore covertly.
The various modes of operating such detection systems can further be used in combination with each other in order to accomplish effective target classification and identification. For example, targets often appear different in different spectral regions, and the degree of difference can be used to distinguish one type of target from another. Some problems in multimode systems are due to the relationship between the various modes and the implementation of these modes in a system. Many such problems faced in implementing multimode multi-spectral systems are inherent in the refractive materials used for the radome (as well as the lens and scanning system) which must permit unhampered egress and ingress of the required bands of radiation.
Some materials, for example, which are transparent in one band may be opaque or only semitransparent in others, and other materials may be transparent in all bands of interest, but subject to different refractive indices and/or degrees of birefringence for each band. Furthermore, materials which may be suitable optically for one or even for all bands of interest, may be unsuitable for use in a radome, because they cannot survive extended exposure to harsh environments, such as those experienced by a radome flying long distances through rain or the like.
Chemical attack (e.g., by humidity, smoke, etc.) and high temperature (induced by atmospheric friction at high speeds, for example) are also of concern for radomes. However, rain erosion is a particularly serious and well-known problem for zinc sulfide and zinc selenide, which are relatively soft but otherwise excellent materials for the 8-12 micrometer infrared band. The mechanism of rain erosion involves impact at high relative velocity between the radome and the raindrops. The consequent microfracturing of the surface increases reflective losses and causes any antireflection coatings applied to the radome to lose their adherence. (By inference, abrasives such as dust and sand would be of similar concern). Rain-erosion-resistant coatings are under development in an effort to surmount this problem, but to date, such coatings have provided only a modest degree of protection under specific sets of test conditions; they cannot protect a radome throughout the entire flight duration and under conditions of potential environmental severity envisioned for a multi-spectral radar/IR antenna.
Enemy countermeasures including laser radiation may also inflict damage on radomes or on the sensor behind them. Laser radiation to which the radome is opaque may raise the radome temperature high enough to cause it to melt or crack for example. Laser radiation to which the radome is transparent may damage the sensor scanner or may even be focussed by the lens onto the detector.
The 8-12 micrometer band is one of two particularly significant infrared spectral regions in which Earth's atmosphere is transparent; the other is the 3-5 micrometer region. For the latter, a commonly accepted material is sapphire, a crystalline form of alumina, which has excellent resistance to abrasion, moisture, and high temperature. Sapphire is slightly birefringent at visible wavelengths and in the 3-5 micrometer band, but this is of little concern for passive detection of unpolarized infrared radiation. At radar frequencies, however, this birefringence is enormous; it would produce unacceptable alterations of polarization states if sapphire were used for a radome. Polycrystalline alumina "randomizes" this birefringence on a microscopic basis, and is, therefore, more suitable for radar. Unfortunately, this is achieved at the cost of transparency in the infrared and visible regions. However, other forms of alumina are available which are ideal for multi-spectral radomes, since they are not birefringent, but they retain the environmental advantages of sapphire and polycrystalline alumina. Such materials include aluminum oxynitride (ALON), a nitrogen-stabilized alumina, and MgO spinel. Unfortunately, all such forms of alumina are opaque in the 8-12 micrometer infrared regions, which is of great importance in many applications for which the 3-5 micrometer region is inadequate. The same problem exists with magnesium oxide and other environmentally stable materials.